Variable dot keyer



Aug. 13, 1946.

M. G. cRosBY 2,405,876

VARIABLE DOT KEYER Filed June 50, 1943 maur/am 50am! ATTORNEY Patented Aug. 13, 1946 VARIABLE DOT KEYER Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application June 30, 1943, Serial No. 492,854

14 Claims. l

This invention relates to vairable dot keyers for radio transmission of modulated waves. More particularly, the invention relates to a system for converting phase or frequency modulated Waves into square waves having a variable percent mark.

The art of dot keying is quite extensive. Certain typical patents which disclose systems of one kind or another for use in on-and-off keying of modulated waves are as follows: Heising Patent No. 1,655,543; Finch Patent No. 1,887,236; and Kell Patent No. 2,061,734. In all of these patents systems were shown for producing a square wave output. The means employed and the methods disclosed were, however, quite different from what is Shown in the instant application. Th'e technique which I prefer is one which is more comparable with the practice of limiting in an amplifier or detector. Such technique is disclosed for other purposes in my Patent No. 2,263,615, dated November 25, 1941, and also in my Patent No. 2,276,565, dated March 17, 1942.

It is an object of my invention to provide a system of dot keying, or square wave generation, as a result of converting a phase modulated wave so as to produce a square wave of variable percent mark. Such a wave is generally termed a constant frequency variable dot wave.

`Another object of my invention is to provide a system for converting a frequency modulated wave into a square wave having variable frequency and variable dot width.

Still another object of my invention is-to provide a method of wave limiting similar to the aforesaid method, but having a characteristic of variability both of frequency and of dot width.

Still another object of my invention is to provide a modulation system wherein the dot width variation in a square wave output is accompanied by an inverse frequency variation, such that th'e energy in the modulation wave remains substantially constant.

Further objects and advantages of my invention will be brought out in the detailed description to follow. This description is accompanied by a drawing, in which:

Fig. 1 shows a circuit diagram of a preferred embodiment, in which a phase modulated wave is converted by means of electronic devices into a square wave output having a characteristic of constant frequency and variable dot keying;

Fig. 2 sh'ows a somewhat similar circuit arrangement, but modified so as to utilize a frequency modulated wave and to convert it into a square wave having the characteristic of variable frequency and variable dot Width;

Figs. 3 and 4 show wave representations illustrative of the input and output energies applicable to Fig. 1; and

Figs. 5 to 8 inclusive are similar wave representations applicable to the circuit arrangement of' Fig. 2.

Referring first to Fig. 1, I show therein a conventional representation of a carrier source I, the output from which is fed to a phase modulator 3. The modulations are derived from a modulation source 2 which' is connected to the phase modulator 3'. Either a pair of discharge tubes or a single tube having independent electron discharge structures may be employed for controlling a subsequent stage from which the desired square wave output may be derived. The tube envelope 6 contains two triode structures and may, therefore, be considered the equivalent of two triode tubes. One of the triode combinations includes cathode l, one of the anodes 9, and a grid IB. The other triode combination includes the cathode l, the other anode 9 and a grid I2. Grid I0 is controlled by variations in potential across the grid leak resistor I I when influenced by output from the phase modulator 3, the energy from which is impressed across coupling condenser 5. Grid I2 is controlled by carrier frequency energy derived directly from the source I and applied across coupling condenser 4. The input circuit on the right hand side of the tube 6 includes grid I2 and grid leak resistor I3 which is grounded.

Since the two triode structures in tube 6 have substantially a common cathode l, it is a feature of my invention to provide a cathode resistor 8, connecting the cathode I with ground. The cathode 'I is connected to the cathode I 5 in a triode tube I4 which' is disposed in a subsequent stage. Tube I4 also possesses an anode I6 and a control grid I'I, the latter being directly grounded.

The anodes 9 in tube 6 are interconnected and are supplied with positive direct current potential indicated as +B through a resistor I8. A capacitor I9 is connected between the anodes 9 and ground in order to by-pass unwanted frequencies. The output from tube I4 may be utilized by means of a coupling condenser 22 which feeds energy to a load resistor 2 I, the lower end of which is connected to ground and to the minus terminal of the direct current source indicated as B. The output energy may. if desired, be rectified as by means of the diode tube'- 20. This method of rectification and detection'isconventional. Such 3 output may, if desired, convey facsimile or voice signals or other types ofmodulated waves, as is well understood by those skilled in the art.

In the operation of the circuit arrangement.

shown in Fig. 1, it should be noted that the output from the phase modulator 3 is variably phaserelated to the constant frequency output from the carrier source I. This relationship determines the timing and duration of conductive states in the cathode resistor 8. The potential drop through this resistor 8 also controls the tube I4, since the grid II is grounded and the potential on cathode I5 must follow that of the cathode l. If, therefore, the output from the phase modulator 3 is in phase agreement with the output from the carrier source I, then both sides of the twin triode tube become simultaneously conductive and are simultaneously cut-ofi. The result is to produce unidirectional pulses in the cathode resistor 8 which persist for 50% of the time.

When, however, the phase angle between the input potentials applied to grids I and I2 approaches 180, the conductive periods in the cathode' resistor 8 are proportionately shortened and are completely cancelled out at 180V phase displacement. But the alternating voltage applied to 4grid I2 is at the fixed frequency of the carrier source I. Hence, the cathode resistor 8 has nonconductive periods which vary between 50% and 100% of the total time; one non-conductive periodl and an immediately following conductive period always representing a full cycle of the carrier frequency.

The value of the anode potential applied to tube I4 is so chosen in relation to the potential drop across resistor 8 that when either side alone or both sides of the tube 6 are conductive then tube I 4 will be substantially biased to cut-0E. Tube I4 is, therefore, conductive only when both sides of tube 6 are simultaneously blocked. During the cut-olf periods of tube I4 capacitor 22 takes a charge through resistors IB and 2I. The diode tube 20 is also conductive during the charging periods. Capacitor 22 discharges through tube I4 during conductive periods of the latter. The discharging circuit includes the space path of tube I4 and the impedances of resistors 2I and 8, but no Vcurrent flows through tube 2i) during the discharging periods.

The utilization circuit may include a modulator in a radio transmitter, or any other apparatus which it is desired to control by means ofv the square wave output from capacitor 22. The capacitor 32 may or may not be needed, depending upon the working potentials at which the utilization device is driven.

The curves drawn in Figs. 3 and 4 have the same time scale. In Fig. 3 the two sine waves, one represented by a solid line curve and the other by a broken line curve, represent input voltages applied to the respective grids I2 and ID in tube 6. The resultant square-wave shown in Fig. 4 is drawn so that its peaks represent conductive periods in tube I4. These are the periods during which capacitor 22 discharges.

Referring now to Fig. 2, I show certain elements in the circuit arrangement thereof which correspond with those of Fig. 1. They, therefore, bear similar reference numerals. In Fig. 2, however, the modulation source 2 is arranged to feed into a frequency modulator 23, the output from which is fed to the two terminals of a tank circuit consisting of the primary winding in a transformer 24 and a variable capacitor 25. One terminal of the tank circuit is coupled to ground across capacitor 21. The other terminal of the tank circuit is coupled across condenser 4 to the grid I2 in the right hand triode section of tube 6. The grid I0 in the left hand section of tube 0 derives control potentials from the tuned secondary winding of transformer 24. Tuning is obtained by means of the variable condenser 26.

The circuit arrangement of Fig. 2 is arranged to produce variable-frequency-variable-dot keying. The frequency-modulated wave is separated into two components one of which is converted to a frequency-and-phase modulated wave by means of the transformer 24. The primary of transformer 24 is tuned by condenser 25, and the secondary by condenser 26. The phase modulation component is imparted by the transformer to grid I0. The other component, which is only frequency-modulated, traverses capacitor 4 and is applied across the grounded resistor I3 for controlling grid I2. The potentials on the two grids I0 and I2, therefore, have a phase relation to one another which is varied in dependence upon frequency departures from the fixed frequency to which the tank circuits on the two sides of the transformer 24 are tuned.

In the system according to Fig. 2 the circuit parameters may be so chosen as to provide any one of three different modes of operation as follows:

ModeV #l is best understood. by reference to Fig. 5. Here the undistorted frequency modulation component applied to grid I2 is represented by the solid line. The frequency-and-phase modulation component is represented by the broken line. Fig. 6, which is drawn to the same time scale as Fig. 5 shows a square Wave the marking peaks of which are co-extensive with the complete cut-off periods in tube 6, during which periods tube I4 is conductive and capacitor 22 discharges. At the frequency to which the tank circuits on the two sides of transformer 24 are tuned a phase displacement of exists in the output from this transformer. The square wave peaks then occupy 25% of the total time. As the frequency is increased the phase displacement of the transformer output component approaches and the percentage of marking time of the square Wave peaks approaches zero as a limit. When the frequency swings below the resonant frequency of the tank circuits, then the phase displacement of the transformer output component approaches 0 and hence the percentage of marking time approaches 50% as a limit.

Operating mode #2 with respect to the system of Fig. 2 is also illustrated by Figs. 5 and 6, but it is a specific adjustment falling within the general range of adjustments covered by mode #1. Here the frequency variation and the phase rotation are so related that the average energy in the square wave peaks, or dots, remains constapt. One object to be attained in this mode of operation is to provide a measure of secrecy in the transmission of signals. Any decrease in the dot length is exactly compensated for by an increase in the number of dots per second.

Special detecting means not commonly used would, therefore, be required in the receiving system in order to translate the signals into intelligence. Such means might not be available at an unauthorized receiving station.

Operating mode #3 is just the reverse of mode #1. This mode is illustrated in Figs. 7 and 8. At the frequency of the tank circuits on the two sides of transformer 24, the output from this transformer bears a 270 phase Yrelation to the as the frequency decreases the phase displacement approaches 180 as a limit'. Hence the dot length varies between 50% and 0%, but vanishes at the low frequency end of the spectrum andV is a max iinum at the high frequency end.

One advantage of operating the system according to mode #3 is that the composite modulation: produces extremes of vari-ation in the energy content of theA signals which have wider limits than when the phase alone is varied. In other words,

the lower the frequency', the more the' marking dots will be reduced inv width, and the higher the frequency, the' more closely will the marking dots approach 50 In order to render theV circuit of Fig. 2 selectively operative according to either mode #l or kmode #3 it is only necessary to choose the proper terminal of the secondary winding in transformer i 24to be coupled across capacitor 5to grid Ill. Reversal of this connection with respect to' the ground connection of the remaining secondary terminal obviously rev'erses the phase of the input potentials applied to grid I0.

In the circuit of Fig. 2 there is no need for the rectifier tube 20 of Fig, l. A bias is applied to the grid I1 as by means of a potentiometer 28, one end of which is grounded and the other end connected to a source of negative bias potential indicated as -C. This source has its positive terminal connected to ground, as indicated by the reference +C. In the operation of tube |14, as shown in Fig. 2, one half of the output wave is cut-off. The square-wave output is, therefore, unidirectional and the positive peaks represent marks of variable width. This output appears across the resistor I8 to which the utilization device is coupled across capacitor 22.

While I have described and shown two embodiments of my invention, it is to be clearly understood that these embodiments are only illustrative and that other modifications within the scope of the invention may be made.

What I claim is:

1. The method of signaling which includes comparing the phase of half-cycles of a phase modulated wave with half-cycles of an umnodulated component of said wave, producing a square wave the positive components of which vary in duration between 50% for the condition of phase agreement between said half-cycles and 0% for the condition of phase opposition, and transmitting a carrier wave which is interrupted by halfcycle components of said square wave.

2. The method of signaling which includes comparing the phase of half-cycles of a frequencymodulated wave with half-cycles of a phasemodulated derivative of said wave, producing a square wave the positive components of which vary in duration between 50% for the condition of phase agreement between said half-cycles and 0% for the condition of phase opposition, and transmitting a carrier wave which is interrupted by half-cycle components of said square wave.

.-3. Apparatus for producing a square wave the marking elements of which Vary in duration be.. tween 50% and 0% in accordance with the degree of overlap of two sine waves, comprising means for frequency-modulating one of said sine waves, means for deriving the second of said sine waves from the first by phase modulation thereof, the phase displacement being a direct function of the frequency variation, means for separately controlling each of two space discharge paths" by a respective one of said sinek waves, means for combining' the currents which flow in the twov said paths, and means vfor so controlling the cur'-I rent in a third discharge path as to 'render the peak-s of said square wave coextensive with periods of current cessation in both of the two dis-- charge paths first-mentioned.

e. Apparatus for producing a square wave the marking elements of which vary in duration between 5'0'% and 0% in accord-ance with the degree of overlap of two sine waves, comprising means for frequency-modulating one' of said sine waves, means for deriving the second of said sine waves from the first by phase modulation thereof, the phase displacement being an inverse function of thel frequency variation, means for separately controlling'each of two space discharge paths by by a respective one of said sine waves, 'means for combining the currents which flow in the two said paths, and means for so controlling the current ina third discharge path as to render the peaks of said square wave coextensive with peri-YV ods of current cessation in both of the two dis-r charge paths first mentioned. l

5. A signaling system comprising a carrier wave source', a source of signals, means controlled by said signals for phase-modulating an output component from said carrier source, amplifier means comprising two discharge paths and a common `cathode resistor, means for separately controlling the two said discharge paths, one'by potentials derived from the unmodulated carrier source, and the other by ypotentials derived from the phase-modulated carrier wave, and electronic means having a square wave output whose wave crests are coextensive in time with periods of zero potential drop in said cathode resistor.

6. A system in accordance with claim v5 and including means for rectifying a component of the output from said electronic means.

7. A signaling system comprising a frequency modulated Wave source, means for phase-modulating an output component from said source,

amplier means comprising two discharge pathsY and a common cathode resistor, means for separately controlling the two said discharge paths, one by potentials derived from the frequency modulated wave source itself and the other by potentials derived from the phase-modulated component thereof, and electronic means having a square wave output whose wave crests are coextensive in time with periods of zero potential drop in said cathode resistor.

8. A system according to claim '7 and including tuned coupling elements in said phase-modulating means, and an input circuit for said other discharge path so arranged as to cause the generation of a square wave the crests of which vary in duration as a direct function of the phase dis- 'placement between the currents in the two said discharge paths.

9. A system according to claim 7 and including tuned coupling elements in said phase-modulating means, and an input circuit for said other discharge path so arranged as to cause the generation of a square wave the crests of which vary in duration as an inverse function of the phase displacement between the currents in the two said discharge paths.

10. A square wave keying system comprising a pair of electron discharge devices having a common cathode resistor connected to ground, each said device having at least the usual electrodes of a triode, means for controlling the respective grids of said devices by signal potentials which are variably phase-related to each other, a thirdy discharge device having cathode, anode and control grid electrodes, the cathodes of al1 three devices being interconnected, a ground connection for the grid in said third device, a load impedance connected between the anode of said third device and a source of anode potential, and means for causing said third device to function as a limiter tube the output from which has a squarewave characteristic the Wave crests whereof are commensurate with periods of zero potential drop in said cathode resistor.

11. A system according to claim 10 and including threshold bias adjusting means applicable to the grid of said third device.

12. A circuit arrangement comprising a carrier source, a modulation source, an electronic device for amplifying a component of output energy from said carrier source, a second electronic device arranged to amplify a component of energy from ,said carrier source after phase modulation there- .of by said modulation source, a third electronic device of the triode type, an anode for the third device connected through a load resistor to an anode potential source. a ground connection for the grid in said third device, a utilization circuit responsive to a square Wave output from said third device, and means for so coupling the cathodes of the three said devices together that the crests of said square wave are commensurate with periodsV of simultaneous cut-off of current flow in the iirst and second of said devices.

13. A circuit arrangement comprising a sourceA 'of frequency modulated waves, a trode discharge device for amplifying an energy component de- 10 rived from said source, a transformer havingv Ituned primary and secondary windings, the primary winding being coupled to said source a 4second triode discharge device arranged to amplify phase rotated energy fed thereto by said. transformer, a common cathode resistor for the two said triode discharge devices and means ineluding a third triode discharge device having its cathode connected to the cathodes of the rst and second discharge devices for producing square `.waves the crests of which are commensurate in time with periods of Zero potential drop in said common cathode resistor.

14. lA circuit arrangement according to claim 13 and including threshold biasing means for the grid in said third discharge device.

MURRAY G. CROSBY.V 

